Convert base::ScopedFD to (mojo::embedder::)ScopedPlatformHandle.

mojo/edk/embedder/platform_handle.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_EDK_EMBEDDER_PLATFORM_HANDLE_H_
 #define MOJO_EDK_EMBEDDER_PLATFORM_HANDLE_H_
 
 #include "build/build_config.h"
 #include "mojo/edk/system/system_impl_export.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
 namespace mojo {
 namespace embedder {
 
+// A |PlatformHandle| is a file descriptor on POSIX and a |HANDLE| on Windows.
 #if defined(OS_POSIX)
 struct MOJO_SYSTEM_IMPL_EXPORT PlatformHandle {
   PlatformHandle() : fd(-1) {}
   explicit PlatformHandle(int fd) : fd(fd) {}
 
   void CloseIfNecessary();
 
   bool is_valid() const { return fd != -1; }
 
   int fd;
@@ skipping 10 matching lines @@
   HANDLE handle;
 };
 #else
 #error "Platform not yet supported."
 #endif
 
 }  // namespace embedder
 }  // namespace mojo
 
 #endif  // MOJO_EDK_EMBEDDER_PLATFORM_HANDLE_H_